Fiber reinforced (matrix) resin composite articles take many shapes and forms in applications ranging from bathtubs to aircraft. Typically in the construction of these shapes and forms fibers are laid into an open mold of the desired shape. This dry fiber reinforcement is then wet out with the matrix resin using manual or instrumented techniques, and the matrix resin is allowed to cure to form the fiber reinforced matrix resin composite to the desired shape. The resulting composite article is then removed from the mold for use.
In circumstances where it is desirous to provide a durable and/or aesthetic surface to the article being manufactured, an in-mold coating capable of forming a primary covalent interfacial bond with the chosen matrix resin system is employed. This in-mold coating is typically applied by manual or instrumented spray techniques onto the mold surface prior to application of the fibers and/or resin. In order to achieve said covalent interfacial bonding the matrix resin is applied to the in-mold coating prior to completion of the in-mold coating's cure. Those fluent in the art will recognize such an in-mold coating as a gelcoat, so named for this pre-cured gel state.
The ability of the gelcoat to provide a primary bond at the critical coating-matrix interface lends a distinct advantage over the application of a coating secondary to the composite article's construction, where bond strength is dictated by the coating system's adhesive qualities. Further, in gelcoated articles aesthetics are derived from the surface quality of the mold in use. Thus, in aesthetically demanding articles, such as boat hulls, aesthetic-finishing labor is associated with the fabrication of the master from which the mold is made. Where multiple like parts are being manufactured from the same mold, this represents a tremendous cost savings over secondary coating where aesthetic-finish labor is associated with the individual article. Other advantages to the use of a gelcoat over a secondary coating are readily apparent to those fluent in the art.
Gelcoats for composite articles are generally multi-component formulations consisting of a base resin system having incorporated therein various fillers, pigments, and other additives. While the selection of these constituents plays an important role in determining the end properties of the gelcoat and its suitability for a given application, the selection of the base resin system dictates the overall end use performance of the gelcoat as a whole. Lending themselves to the requisite demands of durability and aesthetics, to a large extent it is well known that unsaturated ester-based polymers are conventionally utilized as the primary backbone in composite gelcoat systems. These ethylenically unsaturated polyester resins are typically used together with a reactive diluent, usually an unsaturated organic monomer. The unsaturated organic monomers are selected such that they co-polymerize through room temperature free-radical cure with the polyester resin to form the gelcoat. As generally used in the past, exemplar unsaturated organic monomers include styrene, alpha-methylstyrene, vinyltoluene, and divinyl-benzene. Thus gelcoats of this convention are by their nature designed to form interfacial covalent bonds with ethylenically unsaturated matrix resin systems such as polyester and vinylesters.
Although ethylenically unsaturated matrix resins are effectively employed, epoxy matrix resins have a broad range of physical properties, mechanical capabilities and processing conditions that make them invaluable by comparison for many applications. Depending on the chemical structures of the epoxy resin and the co-reactant, modifying reactants chosen, and the conditions of cure, it is possible to obtain toughness, chemical and solvent resistance, mechanical responses ranging from extreme flexibility to high strength and hardness, resistance to creep and fatigue, excellent adhesion to most fibers, heat resistance, low cure related shrinkage, and excellent electrical properties. The uncured epoxy resins have a variety of physical forms, ranging from low-viscosity liquids to tacky solids, which, combined with the large selection of curing agents, affords the composite fabricator a wide range of processing options.
While articles manufactured with epoxy matrix resins may benefit from the use of a gelcoat, previously, covalently compatible in-mold coatings with the requisite durability and aesthetic characteristics has not been available. The mutual exclusivity of the curing mechanisms associated with that of the conventional gelcoat, and more specifically the conventional ethylenically unsaturated monomers used in said gelcoats, and the curing mechanisms of epoxy matrix resins, makes them incompatible. Further, due to extinction of the free radical by oxygen, conventional gelcoats do not cure to completion in atmosphere.
Thus prior art has dictated the use of a tie coat between the conventional gelcoat and the epoxy matrix resin. The tie coat is designed to be covalently compatible with the conventional gelcoat, while curing to completion to provide an interface for secondary bonding of the epoxy matrix resin. Disadvantages associated with the use of said tie coat include increased cost to the final part in application time, increased production cycle time, increased material costs and increased article weight. Further, the mechanical integrity at the critical coating-matrix interface is subject to the adhesive qualities of the epoxy matrix resin employed.
Accordingly, in light of the above discussion, the prior art has faced the problem of finding an acceptable gelcoat for direct application to epoxy matrix fiber-reinforced composites.
In these respects, the use of a gelcoat composition for epoxy (matrix resin) molded plastic (composite) articles according to the present invention substantially departs from the conventional concepts and designs of the prior art.